Method and apparatus for forming a reference pressure within a chamber of a capacitance sensor

ABSTRACT

The present invention is directed at methods and apparatuses for facilitating the establishment of a reference pressure within a reference chamber of a pressure transducer. The transducer has a housing and a cover, the housing defining a reference chamber and an aperture. A meltable sealing material is disposed on at least one of the cover and the housing. The apparatus includes a pressure chamber that is rotatable between a first position and a second position, a pressure source that is connected to the pressure chamber, a guide that is attachable to the transducer near the aperture, and a heater for selectively heating the pressure chamber to a temperature sufficiently high to melt the sealing material. The cover is positioned in an internal space of the guide. The guide is attached to the transducer near the aperture. The transducer, cover and guide are placed in the pressure chamber, the pressure chamber is rotated to the first position and a pressure is generated in the pressure chamber via the pressure source. After a reference pressure has been established in the reference chamber, the pressure chamber is rotated to the second position. Gravity causes the cover to move within the space towards the aperture when the pressure chamber is rotated to the second position. The heater then heats the pressure chamber to melt the sealing material. Upon cooling, the sealing material forms a seal that seals the reference pressure in the reference chamber of the transducer.

CROSS-REFERENCE

This application is a divisional application of U.S. patent applicationSer. No. 10/960,153, filed Oct. 7, 2004.

BACKGROUND OF THE INVENTION

The present invention relates to capacitive pressure transducers. Morespecifically, the present invention relates to an improved method andapparatus for forming a reference pressure within a chamber of acapacitive pressure transducer assembly.

FIG. 1A depicts a cross-sectional side view of an assembled prior artcapacitive pressure transducer assembly 10. FIG. 1B is an exploded viewof the upper housing 40, diaphragm 56 and lower housing 60 of FIG. 1A.Briefly, capacitive pressure transducer assembly 10 includes a body thatdefines an interior cavity. A relatively thin, flexible ceramicdiaphragm 56 divides the interior cavity into a first sealed interiorchamber 52 and a second sealed interior chamber 54. As will be discussedin greater detail below, diaphragm 56 is mounted so that it flexes,moves, or deforms, in response to pressure differentials in chambers 52and 54. Transducer assembly 10 provides a parameter that is indicativeof the amount of diaphragm flexure and this parameter is thereforeindirectly indicative of the differential pressure between chambers 52and 54. The parameter provided by transducer assembly 10 indicative ofthe differential pressure is the electrical capacitance betweendiaphragm 56 and one or more conductors disposed on an upper housing 40.

Capacitive pressure transducer assembly 10 includes a ceramic upperhousing 40 and a ceramic lower housing 60. The upper housing 40, whichgenerally has a circular shape when viewed from the top, defines anupper face 41, a central lower face 47, an annular shoulder 42 that hasa lower face 42 a and an annular channel 43 that is located between thecentral lower face 47 and the annular shoulder 42. Lower face 42 a ofthe annular shoulder 42 is substantially co-planar with central lowerface 47. The upper housing further defines an aperture (or passageway)48 that extends through the housing 40 from the upper side to the lowerside. A metallic conductor 46 is disposed on a center portion of thelower face 47.

The diaphragm 56 is generally a circular thin diaphragm that has anupper face 57 and an opposite, lower, face 59. A metallic conductor 58is disposed on a center portion of upper face 57 of the diaphragm 56.The diaphragm 56 and the upper housing 40 are arranged so that theconductor 46 of the upper housing 40 is disposed opposite to theconductor 58 of the diaphragm 56. Diaphragm 56 is coupled to the upperhousing 40 by a high-temperature air-tight seal (or joint) 70. The seal70 is located between the lower face 42 a of the annular shoulder 42 ofthe upper housing 40 and a corresponding annular portion of face 57 ofdiaphragm 56. When sealed, the upper housing 40, seal 70 and diaphragm56 define reference chamber 52. A reference pressure is established andmaintained in the reference chamber 52. Aperture 48 provides an inlet orentry way into reference chamber 52.

The lower housing 60, which generally has a circular shape, defines acentral opening 64 and an upwardly projecting annular shoulder 62 thathas an upper face 62 a. The upper face 62 a of shoulder 62 of the lowerhousing 60 is coupled to a corresponding portion of lower face 59 ofdiaphragm 56 by a high-temperature air-tight seal (or joint) 76. Seal 76can be deposited and fabricated in a manner similar to that of seal 70.When sealed, the lower housing 60, seal 76 and face 59 of the diaphragm56 define process chamber 54.

A pressure tube 66 having an inlet passageway 68 is coupled to the lowerhousing 60 by a seal, for example, so that the inlet passageway 68 isaligned with the opening 64 of the lower housing 60. Accordingly, theprocess chamber 54 is in fluid communication, via opening 64 and inletpassageway 68, with an external environment. In operation, thecapacitive pressure transducer assembly 10 measures the pressure of thisexternal environment.

Conductors 46 and 58 of the capacitive pressure transducer assembly 10form parallel plates of a variable capacitor C. As is well known,C=Aε_(r)ε₀/d, where C is the capacitance between two parallel plates, Ais the common area between the plates, ε₀ is the permittivity of avacuum, ε_(r) is the relative permittivity of the material separatingthe plates (e.g., ε_(r)=1 for vacuum), and d is the axial distancebetween the plates (i.e., the distance between the plates measured alongan axis normal to the plates). So, the capacitance provided by capacitorC is a function of the axial distance between conductor 46 and conductor58. As the diaphragm 56 moves or flexes up and down, in response tochanges in the pressure differential between chambers 52 and 54, thecapacitance provided by capacitor C also changes. At any instant intime, the capacitance provided by capacitor C is indicative of theinstantaneous differential pressure between chambers 52 and 54. Knownelectrical circuits (e.g., a “tank” circuit characterized by a resonantfrequency that is a function of the capacitance provided by capacitor C)may be used to measure the capacitance provided by capacitor C and toprovide an electrical signal representative of the differentialpressure. Conductors 46, 58 can be comprised of a wide variety ofconductive materials such as gold or copper, for example, and can befabricated via known thin and thick film processes or other knownfabrication methods. When thin film processes are utilized, conductors46, 48 may have thicknesses of about 1 μm, for example.

Diaphragm 56 is often made from aluminum oxide. Other ceramic materials,such as ceramic monocrystalline oxide materials, however, may also beused. Capacitance sensors having ceramic components are disclosed inU.S. Pat. Nos. 5,920,015 and 6,122,976.

As noted above, changes in the differential pressure between chambers52, 54 cause diaphragm 56 to flex thereby changing the gap betweenconductor 46 and conductor 58. Measurement of changes in the gap permitsmeasurement of the differential pressure. The gap, however, can also beaffected by factors unrelated to pressure. For example, the gap can beaffected by changes in temperature. Since the components of transducerassembly 10 can be made from a variety of different materials, each ofwhich has its own characteristic coefficient of thermal expansion,temperature changes in the ambient environment can cause the diaphragm56 to move closer to, or further away from, conductor 46. Fortunately,changes in the gap caused by temperature changes are characteristicallydifferent than changes in the gap caused by changes in differentialpressure. To compensate for changes in the gap that are caused due tochanges in the ambient temperature, it is known to include a secondconductor (not shown) that is disposed adjacent to conductor 46 on thelower face 47 of the upper housing 40. In such an embodiment, conductors46 and 58 form parallel plates of a variable capacitor C1 and conductor58 and the second conductor form parallel plates of a variable capacitorC2. The two capacitors, C1 and C2, may be used by known methods toreduce the transducer's sensitivity to temperature changes.

The upper housing 40 is positioned so that the lower face 47, and anyconductors disposed thereon, are disposed in a plane that is parallel tothe plane defined by the conductor 58 (i.e., diaphragm 56) when thepressures in chambers 52, 54 are equal. As discussed above, thecapacitance defined by the conductors 46, 58 depends upon the gap (i.e.,axial distance) that exists between these opposing conductors. The gap,which is relatively small (e.g., on the order of 0.0004 inches (10-12μm)), depends, in part, upon the thickness of the seal 70 and the shapeand configuration of the upper housing 40 (e.g., the amount that lowerface 42 a is out of plane, i.e. offset, with lower face 47, if any).

In operation, capacitive pressure transducer assembly 10 is normallyused as an absolute pressure transducer. In this form, reference chamber52 is evacuated to essentially zero pressure, e.g., less than 10⁻⁸ Torr,and the reference chamber 52 is then sealed. The reference pressure thenserves as a baseline from which a pressure within the process chamber 54is determined. To maintain the essentially zero pressure within thereference chamber 52, the transducer assembly 10 includes a tube 80, acover 82, a hold-wire 86, a screen 88 and a getter element 84. As isshown in FIGS. 1A and 1B, the screen 88 supports the getter element 84within a hollow portion of the tube 80 while the hold-wire 86 maintainsthe getter element 84 against the screen 88. The hollow portion of thetube 80 is disposed over the aperture 48 of the upper housing 40 so thatthe getter element 84 is in fluid communication with the referencechamber 52. In addition to supporting the getter element 84, screen 88also prevents particles from passing into the reference 52 that couldadversely affect the operation of the diaphragm 56.

The bottom end of the tube 80 is coupled to the upper face 41 of theupper housing 40 around the aperture 48 by a high-temperature air-tightseal 92, while the cover 82 is coupled to the upper end of the tube 80by a low-temperature air-tight seal 94. Seals 92, 94 and seal 70, whichis located between the shoulder 42 of the upper housing 40 and thediaphragm 56, all assist in maintaining the reference pressure that isestablished in the reference chamber 52. The high-temperature seal 92 iscomprised of a high-temperature glass material while the low-temperatureseal 94 is comprised of a low-temperature glass material. To form thehigh-temperature seal 92, the high-temperature glass material isdeposited on the lower end of the tube 80, a corresponding sealing areaof face 41, or both. The high-temperature glass material is melted, aforce perpendicular to the upper face 41 of the upper housing 40 isapplied between the tube 80 and the upper housing 40 and thehigh-temperature glass material is then allowed to cool (i.e., solidify)thus forming the high-temperature air-tight seal 92. The low-temperatureseal 94 is similarly formed between the upper end of the tube 80 and acorresponding sealing area of the cover 82. The high-temperature glassmaterial of the high-temperature seal 92 has a melting temperature thatis higher than that of the low-temperature glass material of thelow-temperature seal 94. To provide different melting temperatures, theglass materials of the seals 92, 94 can be comprised of differentmaterials or have different amounts of a common material. The meltingtemperature of the high-temperature seal 92 is higher than the meltingtemperature(s) of the high-temperature seals 70 and 76 and the meltingtemperature(s) of the high-temperature seals 70 and 76 is higher thanthe melting temperature of the low-temperature seal 94.

The getter element 84 is comprised of a material that, when activated,acts to effectively absorb any gaseous impurities that may be presentwithin the sealed reference chamber 52. Thus, when activated, the getterelement 84 assists in maintaining the reference pressure at an ultrahigh vacuum level for long periods of time, e.g., ten or more years.

Although an ultra high vacuum pressure, i.e., essentially zero pressure,is a convenient and useful reference pressure, other reference pressurescan also be used. After the reference pressure has been established inchamber 52, the pressure tube 66 is then connected to a source of fluid(not shown) to permit measurement of the pressure of that fluid.Coupling the pressure tube 66 in this fashion delivers the fluid, thepressure of which is to be measured, to process chamber 54 (and to thelower face 59 of the diaphragm 56). The center of diaphragm 56 moves orflexes up or down in response to the differential pressure betweenchamber 52 and 54 thereby changing the capacitance of capacitor C. Sincethe instantaneous capacitance of capacitor C is indicative of theposition of the diaphragm 56, transducer assembly 10 permits measurementof the pressure in chamber 54 relative to the reference pressure that isestablished in chamber 52.

The accuracy of the capacitive pressure transducer assembly 10 candepend upon the accuracy at which the reference pressure can beestablished and maintained in the reference chamber 52. In other words,as the actual pressure within the reference chamber 52 deviates from anintended and designed reference pressure, the performance of thecapacitive pressure transducer assembly 10 will correspondingly suffer.

The steps of establishing a reference pressure in the reference chamber52, activating the getter element 84 and sealing the cover 82 to thetube 80 are typically the last few steps that are performed whenfabricating capacitive pressure transducer assembly 10. Thus, the stepsof coupling the upper housing 40 to the diaphragm 56 via thehigh-temperature seal 70, coupling the lower housing 60 to the diaphragm56 via the high-temperature seal 76, coupling the pressure tube 66 tothe lower housing 60 around the opening 64, and coupling the tube 80(having the screen 88, getter element 84 and hold-wire 86) to the face41 of the upper housing 40 around the aperture 48 via thehigh-temperature seal 92 will usually have already been completed beforethe reference pressure is established.

To establish a reference pressure within the reference chamber 52, thereference chamber 52 is typically subjected to a burn-out and evacuationprocess and then the cover 82 is sealed to the tube 80. The referencechamber 52 is “burned-out” by heating the inner surfaces that define thereference chamber 52 (including the surfaces of the cover 82, tube 80,housing 40 that are in fluid communication with the reference chamber52), and the chamber 52 is “evacuated” by drawing an ultra-high vacuumon the reference chamber 52. The burn-out heat vaporizes thecontaminants, e.g., volatiles, moisture, that may be present on theseinner surfaces while the evacuation vacuum draws the vaporizedcontaminants and gases out of the reference chamber 52. Since the cover82 has not yet been sealed to the tube 80, the contaminants and gasesare sucked out of the reference chamber 52, the aperture 48 and thehollow portion of the tube 80. Once the burn-out and evacuation processis completed and while the vacuum pressure is continuing to bemaintained, the cover 82 is then sealed to the tube 80 via thelow-temperature seal 94 to establish the reference pressure in thereference chamber 52.

FIGS. 2A and 2B illustrate a prior art method and apparatus that is usedto establish a reference pressure within the reference chamber 52 of acapacitive pressure transducer assembly 10. FIG. 2A generally depictsthe burn-out and evacuation process while FIG. 2B generally depicts theprocess by which the cover 82 is sealed onto the upper end of the tube80. The apparatus includes a vacuum housing 93 that defines an interiorvacuum chamber 95. Referring to FIG. 2A, a low-temperature sealingmaterial 94 a is deposited on the upper end of the tube 80. Thesemi-completed transducer assembly 10, i.e., one that does not yet havethe cover 82 sealed to the tube 80, is then disposed in the vacuumchamber 95. [For clarity, the pressure tube 66 has been omitted fromFIGS. 2A and 2B and some of the other subsequent figures. The pressuretube 66, however, would typically be coupled to the lower housing 60prior to the assembly being placed in the vacuum chamber 95.] After thetransducer assembly 10 has been placed in the vacuum chamber 95, thevacuum housing 93 is placed in an oven (not shown), a vacuum source (notshown) is coupled to the vacuum chamber 95 and the burn-out andevacuation process of the reference 52 is initiated. During the burn-outand evacuation process, which can last for more than 20 hours, thetransducer assembly 10 is heated to a temperature of about 250° C. andan ultra-high vacuum pressure of the order of 10⁻⁸ Torr (or less) isgenerated in the vacuum chamber 95. In FIG. 2A, the burn-out andevacuation of reference chamber 52 (and aperture 48 and tube 80) isindicated by the arrows which extend from the reference chamber 52, upthrough the aperture 48 and up through and out of the top end of thetube 80.

After the burn-out and evacuation of the reference chamber 52 iscompleted, the cover 82 is then coupled to the tube 80 by thelow-temperature seal 94. Cover 82 is attached and sealed to the tube 80without opening vacuum housing 93 so as to preserve the vacuum inreference chamber 52. Accordingly, as can been seen in FIG. 2A, prior toinitiating the burn-out and evacuation process, the cover 82 is attachedto an end of a rod 96 which penetrates into the vacuum chamber 95 of thevacuum housing 93. When the burn-out and evacuation process iscompleted, the rod 96 can be actuated to bring the cover 82 in contactwith the low-temperature sealing material 94 a that is disposed on theupper end of the tube 80.

The low-temperature sealing material 94 a that forms the low-temperatureseal 94 is not melted during the burn-out and evacuation process, i.e.,the burn-out temperature is generally set below the melting temperatureof the low-temperature sealing material 94 a. Moreover, the burn-out andevacuation process should not compromise the seals that have alreadybeen formed in the transducer assembly 10 (e.g., high-temperature seals70, 76 and 92) and, thus, the burn-out temperature should not exceed themelting temperatures of these seals.

A high-temperature dynamic seal 99 (e.g., a gasket) is disposed in thevacuum housing 93 where the rod 96 penetrates the vacuum housing 93. Thehigh-temperature dynamic seal 99 allows to the rod to travel freely upand down while assisting to maintain the pressure that is present in thevacuum chamber 95 of the vacuum housing 93.

Prior to initiating the burn-out and evacuation process, cover 82 isattached to the end of the rod 96 by a low-temperature seal 98. Themelting temperature (i.e., melting point) of the low-temperature seal98, which is lower than the melting temperature of the low-temperaturesealing material 94 a, is higher than the burn-out temperature and,therefore, does not melt during the burn-out and evacuation process. Therod 96 extends through the high-temperature dynamic seal 99 and,together with the cover 82, is aligned with the tube 80 of thetransducer assembly 10.

Referring now to FIG. 2B, after the burn-out and evacuation process iscompleted, while the pressure in the vacuum chamber 95 is still beingmaintained, the rod 96/cover 82 is lowered until the cover 82 comes intocontact with the low-temperature sealing material 94 a. The temperaturewithin the vacuum chamber 95 (as directed by the oven) is then elevatedto cause the low-temperature sealing material 94 a to melt. Thisincrease in temperature also causes the low-temperature seal 98 to meltand causes the getter element 84 to become activated. To form thelow-temperature air-tight seal 94 between the cover 82 and the tube 80,the temperature within the vacuum chamber 95 is decreased until thelow-temperature sealing material 94 a solidifies and, while thelow-temperature seal 98 is sufficiently melted, the rod 96 is pulledaway from the transducer assembly 10. Once the low-temperature seal 94is formed—and the reference pressure in the reference chamber 52 is thusestablished—the temperature in the vacuum chamber 95 is reduced toambient temperature, then vacuum source is disconnected and theassembled transducer assembly 10 is removed from the vacuum housing 93.

FIG. 3 illustrates the prior art burn-out, evacuation and sealingprocess of the apparatus and method of FIGS. 2A and 2B in more detail.In FIG. 3, the x-axis of the process flow represents Time and the y-axisrepresents Temperature in degrees Celsius. Prior to initiating theburn-out and evacuation process, at Step A of the process flow, thecover 82 is attached to rod 96 via low-temperature seal 98 and thetransducer assembly 10, cover 82 and rod 96 are placed in the vacuumchamber 95 (FIG. 2A). During Step A→B, the temperature in the vacuumchamber 95 is raised to a burn-out temperature of 250° C. and thepressure is lowered to an evacuation pressure of 10⁻⁸ Torr. Step A→B iscompleted in three hours. After the burn-out temperature and evacuationpressure are achieved (Step B), the reference chamber 52 is burned-outand evacuated for 20 hours, Step B→C. Shortly before Step C is reached,the rod 96 and cover 82 are lowered so that the cover 82 comes intocontact with the low-temperature sealing material 94 a that is depositedon the upper end of the tube 80. Once the burn-out and evacuation stepis completed (Step C), the temperature in the vacuum chamber 95 israised to 475° C., Step C→D, which causes the low-temperature sealingmaterial 94 a and the low-temperature seal 98 to melt. Step C→D lastsfor three hours. The vacuum chamber 95 is then maintained at 475° C. for30 minutes, Step D→E, to ensure that the low-temperature sealingmaterial 94 a and the low-temperature seal 98 are sufficiently melted.The temperature in the vacuum chamber 95 is then lowered to 400° C. overthe course of two hours, Step E→F, which causes the low-temperaturesealing material 94 a to solidify and form the low-temperature air-tightseal 94. The melting temperature of the low-temperature seal 98 is below400° C. and, thus, the low-temperature seal 98 remains melted throughoutStep ELF. Shortly before Step F is reached, rod 96 is raised away fromthe cover 82 (FIG. 2B). Lastly, the temperature and pressure in thevacuum chamber 95 are brought to ambient conditions over the course of 4hours and the assembled pressure transducer assembly 10 is then removedfrom the vacuum chamber 95 of the vacuum housing 93, Step F→G. Asillustrated in FIG. 3, the prior art burn-out, evacuation and sealingprocess can be completed in 32 hours.

The method and apparatus described above does not necessarily ensurethat an accurate reference pressure has been established within thereference chamber 52 of a capacitive pressure transducer assembly 10.For example, it is very difficult to establish and maintain an ultrahigh vacuum of the order of 10⁻⁸ Torr (or less) in a vacuum housing 93that utilizes a rod 96 and a high-temperature dynamic seal 99 becausethe pressure integrity of the vacuum housing 93 tends to be compromisedby the presence of the high-temperature dynamic seal 99. It also can bedifficult or costly to accurately control the positions and orientationsof the cover 82 and the tube 80 during the rod actuating mating process.If the cover 82 is not positioned or oriented properly in relationshipto the tube 80 during the mating process, the integrity of thelow-temperature seal 94 may be compromised or the low-temperature seal94 may fail entirely.

A need therefore exists for a method and apparatus for accuratelyestablishing a reference pressure within a reference chamber of acapacitive pressure transducer assembly.

SUMMARY OF THE INVENTION

The present invention is directed to methods and apparatuses forestablishing a reference pressure within a reference chamber of acapacitive pressure transducer assembly.

The pressure transducer includes a cover and a housing that defines areference chamber and an aperture. A meltable sealing material isdisposed on at least one of the cover and the housing. The apparatusincludes a pressure chamber that is rotatable between a first positionand a second position and a guide that is attachable to the transducernear the aperture. The guide defines an internal space. A cable can beused to rotate the pressure chamber between the first and secondpositions. An actuator motor and an actuator rod can alternatively beused to rotate the pressure chamber. A pressure source connected to thepressure chamber can establish a desired pressure within the pressurechamber while a heater (e.g., oven) can selectively heat the pressurechamber to a temperature sufficiently high to melt the sealing material.

The cover is positioned in the space of the guide and the guide isattached to the transducer near the aperture. The transducer, cover andguide are placed in the pressure chamber, the pressure chamber isrotated to the first position and a pressure is generated in thepressure chamber via the pressure source and the chamber is heated tobake out unwanted materials. After a reference pressure has beenestablished in the reference chamber, the pressure chamber is rotated tothe second position wherein gravity thereby causes the cover to movetowards the aperture within the space. The heater then heats thepressure chamber to melt the sealing material. Upon cooling, the sealingmaterial forms a seal that seals the reference pressure in the referencechamber of the transducer.

The apparatus may also include a weight, such as a ball, that isdisposed within the space of the guide.

By utilizing an apparatus that has a guide and a rotatable pressurechamber, the methods and apparatuses of the present invention arecapable of accurately locating and orienting the cover during thereference chamber sealing process. The methods and apparatuses of thepresent invention, moreover, do not require the use of ahigh-temperature dynamic seal to maintain the pressure in the pressurechamber.

BRIEF DESCRIPTION OF THE DRAWINGS

Various objects, features, and advantages of the present invention canbe more fully appreciated with reference to the following detaileddescription of the invention when considered in connection with thefollowing drawing, in which like reference numerals identify likeelements. The following drawings are for the purpose of illustrationonly and are not intended to be limiting of the invention, the scope ofwhich is set forth in the claims that follow.

FIG. 1A shows a cross-sectional view of a prior art capacitance sensor.

FIG. 1B shows partial, expanded cross-sectional view of the prior artcapacitance sensor of FIG. 1A.

FIGS. 2A and 2B illustrate a prior art method and apparatus used toestablish a reference pressure within a reference chamber of acapacitive pressure transducer assembly.

FIG. 3 depicts a process flow for establishing a reference pressurewithin a reference chamber of a transducer assembly in accordance withthe prior art method and apparatus of FIGS. 2A and 2B.

FIG. 4A shows a side view of an apparatus constructed in accordance withthe invention for establishing a reference pressure within a referencechamber of a capacitive pressure transducer assembly.

FIG. 4B shows a front view of the apparatus of FIG. 4A.

FIG. 5 illustrates a partial, cross-sectional, side-view of theapparatus of FIGS. 4A and 4B that shows the internal components of theapparatus and how the pressure transducer assembly is disposed therein.

FIG. 6A shows a frame assembly constructed in accordance with theinvention.

FIG. 6B shows a cross-section view of the frame assembly of FIG. 6A.

FIG. 7A illustrates one step in an exemplary method of establishing areference pressure within a reference chamber of a capacitive pressuretransducer assembly in accordance with the invention.

FIG. 7B is a close-up view that further illustrates how the step of FIG.7A is to be performed.

FIG. 8A illustrates another step in an exemplary method of establishinga reference pressure within a reference chamber of a capacitive pressuretransducer assembly in accordance with the invention.

FIG. 8B is a close-up view that further illustrates how the step of FIG.8A is to be performed.

FIG. 9 illustrates a process flow for establishing a reference pressurewithin a reference chamber of a transducer assembly in accordance withthe method and apparatus of the present disclosure.

DETAILED DESCRIPTION

The present invention is directed to methods and apparatuses foraccurately establishing a reference pressure within a reference chamberof a capacitive pressure transducer assembly. The present invention iscapable of establishing an ultra-high vacuum in a vacuum chamber forfacilitating the burn-out and evacuation process of a transducerassembly and is capable of controlling the delivery and mating of anaperture cover during the reference chamber sealing process. Moreover,the present invention does not utilize a high-temperature dynamic sealto maintain the ultra-high vacuum in the vacuum chamber.

FIG. 4A depicts a side view of an exemplary apparatus 100 constructed inaccordance with the invention. FIG. 4B depicts a front view of theapparatus 100. Apparatus 100 is comprised of a vacuum housing 110 and asupport assembly 120. The vacuum housing 110 defines an internal vacuumchamber, which is discussed in more detail below. A capacitive pressuretransducer assembly 10 that is to be burned-out, evacuated and sealed issecured within the internal vacuum chamber of the vacuum housing 110.The support assembly 120 supports the vacuum housing 110 when thecapacitive pressure transducer assembly 10 is being burned-out,evacuated and sealed and, more specifically, allows the vacuum housing110 and the capacitive pressure transducer assembly 10 that is disposedtherein to be rotated while these processing steps are being performed.

The vacuum housing 110 includes a metal lower flange 112 and a metalupper housing 114. The vacuum housing 110 also includes left and rightpins 136 that are coupled to the upper housing 114 and a vacuum port(not shown) that can be connected to one end of a vacuum line 138. Theother end of the vacuum line 138 is connected to a vacuum pump (notshown) that is capable of drawing an ultra-high vacuum. The pins 136define a rotational axis 240 through which the vacuum housing 110 canrotate when supported by the support assembly 120. The vacuum port islocated near the left pin 136, i.e., near the rotational axis 240, sothat the vacuum line 138 is subjected to a minimum amount ofdisplacement and flexure when the vacuum housing 110 is rotated. Thevacuum housing 110 also includes a cable (or wire) 116 having an endthat is coupled to the backside of the lower flange 112. When the vacuumhousing 110 is secured in the support assembly 120, i.e., via the pins136, the cable 116 can be operated to rotate the vacuum housing 110forward to a downwardly-slanted position (FIGS. 7A and 7B) and backwardsto an upright position (FIGS. 8A and 8B).

The support assembly 120 includes a base 126, left and right supportbrackets 132, two lower supports 122 and two upper supports 124. Thesupport brackets 132, lower supports 122 and upper supports 124 are allmounted on a face of the base 126. The base 126 includes a front edge, aback edge and opposite side edges. As can be seen in FIGS. 4A and 4B,the support brackets 132 are located near the opposite side edges of thebase 126, the upper supports 124 are located inboard of the supportbrackets 132 near the back edge of the base 126 while the lower supports122 are located inboard of the upper supports 124 near the front edge ofthe base 126. Each support bracket 132 has a slot (or hole) 134 that canaccommodate a pin 136. Each lower support 122 has a distal end 122 a andeach upper support 124 has a distal end 124 a. The vacuum housing 110 issecured in the support assembly 120 by mounting the pins 136 of theupper housing 114 into the slots 134 of the support brackets 132. Theslots 134 can be slotted and indexed to accommodate the pins 136 and tofacilitate the rotation and loading and unloading of the vacuum housing110.

After the capacitive pressure transducer assembly 10 has been placed inthe vacuum housing 110 and the vacuum housing 110 has been secured tothe support assembly 120, the apparatus 100 is placed in an oven (notshown) and the vacuum line 138 is coupled to the vacuum port. To operatethe cable 116 at a location that is external to the oven, the oppositeend of the cable 116 is routed between the support brackets 132 and outthrough an access port that is provided in the oven.

As is discussed in more detail below, when the vacuum housing 110 is inan upright position (as shown in FIGS. 4A and 4B), the lower flange 112of the vacuum housing 100 rests upon the distal ends 124 a of the uppersupports 124. However, when the vacuum housing 110 is rotated forward(as shown in FIGS. 7A and 7B), the upper housing 114 then comes to reston the distal ends 122 a of the lower supports 122.

FIG. 5, which depicts a cross-sectional, side-view of the vacuum housing110, shows some additional components of the apparatus 100 andillustrates how the capacitive pressure transducer assembly 10 issecured in the vacuum housing 110. As can be seen in FIG. 5, the vacuumhousing 110 also includes a copper sensor support 210 that secures thetransducer assembly 10 that is to be burned-out, evacuated and sealed.The transducer assembly 10 can be secured to the sensor support 210 bytightening screws (not shown) or by a wide variety of other types offastening means that are suitable for temporarily securing thetransducer assembly 10 to the sensor support 210. The transducerassembly 10 that is to be secured to the sensor support 210 generallyhas a low-temperature sealing material 94 a deposited on the upper endof the tube 80 and on the corresponding sealing surface of the cover 82.

The apparatus 100 further includes a cylindrical guide assembly 300, aball 320 and copper wool 330. The ball 320 is disposed within a hollowportion of the guide assembly 300. As is discussed in more detail below,the guide assembly 300 is temporarily coupled to the tube 80 and,together with the ball 320, guides the cover 82 towards the upper end ofthe tube 80 during the sealing process. The ball 320 is comprised of ahigh-temperature, high-density material such as Tungsten Carbide orSilicon Nitride, for example. The copper wool 330, which is disposedbetween the guide assembly 300 and the upper housing 114, provides athermal conductive pathway between the upper housing 114, the guideassembly 300 and the transducer assembly 10.

After the transducer assembly 10 has been secured in the sensor support210, the sensor support 210 is coupled to the lower flange 112, the ball320, guide assembly 300 and copper wool 330 are installed and the lowerflange 112 is then coupled to the upper housing 114. When assembled, thelower flange 112 and upper housing 114 define an interior vacuum chamber200.

To ensure that the vacuum chamber 200 is air-tight, a temporaryair-tight copper seal is provided between the lower flange 112 and theupper housing 110. The vacuum port (not shown) provides fluidcommunication between the vacuum line 138 and the vacuum chamber 200.During the burn-out, evacuation and sealing steps, the external vacuumpump evacuates the vacuum chamber 200 to an ultra-high vacuum pressurevia the vacuum line 138 and vacuum port.

FIG. 6A shows the cylindrical guide assembly 300 in more detail, whileFIG. 6B shows a cross-section view of the guide assembly 300 and how theball 320 is disposed within the hollow portion of the guide assembly300. The cylindrical guide assembly 300 defines a hollow cylindricalinterior space 316 having a closed distal end 312 and an open proximalend 314. The ball 320 is disposed within the space 316 of the guideassembly 300 and, depending upon the orientation of the guide assembly300, can move freely towards or away from the distal end 312 and theproximal end 314 of the guide assembly 300. To prevent excessiveside-to-side motions (i.e., motions that are perpendicular to a linethat is drawn between the distal end 312 and the proximal end 314) ofthe ball 320 within the space 316, the diameter of the ball 320 isclosely matched to the diameter dimension of the space 316, i.e., thediameter of the ball 320 is slightly less than the diameter of the space316. In one exemplary embodiment, for example, the diameter of the ball320 is 0.5000±0.0001 inches and the diameter of the space 316 is0.505±0.002 inches. The diameters of the ball 320 and space 361 areappropriately sized to account for any thermal expansion effects thatmay occur during the burn-out and evacuation process.

The interior space 316 of the guide assembly 300 is also sized andconfigured to accommodate the cover 82 and tube 80 that are temporarilydisposed within the space 316. The tube 80 and cover 82 generally havethe same radial dimension. The radial dimension of the space 316 is,therefore, established to be slightly larger than the radial dimensionsof the cover 82 and tube 80.

The cylindrical guide assembly 300 further includes a set of holes 310that are arranged radially throughout the guide assembly 300 and a setof tightening screws 318 that are disposed towards the proximal end ofthe guide assembly 300. The tightening screws 318 are used totemporarily secure the guide assembly 300 (with the ball 320 disposedtherein) to the tube 80 during the burn-out, evacuation and sealingsteps. The holes 310 provide a fluid pathway between the interior space316 of the guide assembly 300 and the vacuum chamber 200. Thus, duringthe burn-out and evacuation process, i.e., when the cover 82 has not yetbeen sealed on the tube 80, fluid pathways exist between the referencechamber 52 and the vacuum chamber 200 via the aperture 48, hollowportion of the tube 80 and the holes 310.

FIG. 7A is a side view that illustrates how the vacuum housing 110,guide assembly 300 and ball 320 of the apparatus 100 are oriented duringthe burn-out and evacuation process. FIG. 7B shows a close-up, side viewthat more accurately depicts the orientation and arrangement of the tube80, cover 82, guide assembly 300 and ball 320 of FIG. 7A. As previouslydiscussed, prior to securing the transducer assembly 10 into the sensorsupport 210, low-temperature sealing material 94 a is deposited onto theupper end of the tube 80 and the corresponding sealing area of the cover82. After the transducer assembly 10 is secured in the sensor support210 and the vacuum chamber 200 has been sealed and secured in thesupport assembly 120, the vacuum housing 110 is then rotated in acounterclockwise direction (as shown in FIG. 7A), i.e., forward, untilthe vacuum housing 110 comes to rest on the distal ends 122 a of thelower supports 122. The distal ends 122 a are located such that, uponrotation, the ball 320 and cover 82 which are located within theinterior space 316 of the guide assembly 300 travel away from the tube80 towards the distal end 312 of the guide assembly 300. Thus, bysufficiently rotating the vacuum housing 110, one can ensure that a gap(i.e., a fluid pathway) between the cover 82 and the tube 80 is presentduring burn-out and evacuation process. Once the transducer assembly 10has been brought up to the desired burn-out temperature and anultra-high vacuum pressure has been established and is being drawn inthe vacuum chamber 200, the burn-out and evacuation processing of thetransducer assembly 10 is then initiated. As is indicated by the arrowsin FIG. 7B, the reference chamber 52 of the transducer assembly 10 isevacuated by drawing the contaminants and gases out of the assembly 10and into the vacuum chamber 200 via the aperture 48 (not shown), thetube 80 and the holes 310 of the guide assembly 300. The contaminantsand gases are then further drawn out of the vacuum chamber 300 by theexternal vacuum pump via the vacuum port and vacuum line 138.

The cable 116 can be manipulated to cause the vacuum housing 110 torotate counterclockwise. The vacuum housing 110, for example, can beweighted so that a slackening of the cable 116 causes the vacuum housing110 to rotate counterclockwise, i.e., forward.

Once the burn-out and evacuation process has been completed, thereference pressure in the reference chamber 52 is then locked in bysealing the cover 82 to the tube 80. FIG. 8A is a side view thatillustrates how the vacuum housing 110, guide assembly 300 and ball 320of the apparatus 100 are oriented during the cover sealing process. FIG.8B shows a close-up, side view that more accurately depicts theorientation and arrangement of the tube 80, cover 82, guide assembly 300and ball 320 of FIG. 8A. To seal the cover 82 onto the tube 80, thevacuum housing 110 of the apparatus 100 is rotated in a clockwisedirection (as shown in FIG. 8A), i.e., backwards, to an upright positionby pulling the cable 116 that is attached to the backside of the lowerflange 112. Cable guides (not shown), such as pulley wheels or othertypes of devices or guides, can be utilized to facilitate the operationof the cable 116. When the vacuum housing 110 is pulled into its uprightposition, the lower flange 112 of the vacuum housing 110 will come torest on the distal ends 124 a of the upper supports 124 of the supportassembly 120. The upright position need not be exactly vertical.Instead, it may be advantageous to position the distal ends 124 a of theupper supports 124 so that, upon rotation, the vacuum housing 110 leansslightly backwards. That way, if the tension in the cable 116 slackens,the vacuum housing 110 is less likely to inadvertently rotate forwardtowards the distal ends 122 a of the lower supports 122.

When vacuum housing 110 is rotated to its upright position, gravitycauses the ball 320 to move towards the proximal end 314 of the guideassembly 300 which thereby causes the cover 82 to engage the tube 82and, more specifically, causes the low-temperature sealing material 94 athat is disposed on the bottom-side of the cover 82 to come into contactwith the low-temperature sealing material 94 a that is disposed on theupper end of the tube 80. As situated, the weight of the ball 320 andthe weight of the cover 82 thus provide a contact force between thecover 82 and the tube 80 in the area of the low-temperature sealingmaterial 94 a interface. To seal the cover 82 onto the tube 80, i.e., toform the low-temperature seal 94, while the ultra-high vacuum is stillbeing maintained in the vacuum chamber 200 the temperature in the ovenis elevated to cause the two layers of low-temperature sealing material94 a to melt and fuse together. This increase in temperature also servesto activate the getter element 84 that is disposed in the tube 80. Afterthe layers of low-temperature sealing material 94 a have sufficientlymelted and fused together, the temperature is lowered below the meltingpoint of the low-temperature sealing material 94 a and, upon cooling,the low-temperature air-tight seal 94 is thus formed between the cover82 and the tube 80 (FIG. 8B). By blocking the last fluid pathway thatexisted between the reference chamber 52 and the external environment,i.e., the vacuum chamber 200, the reference pressure in the referencechamber 52 is thus established when the seal 94 is formed.

Once the seal 94 is formed, the oven and vacuum pump can be turned offand the completed transducer assembly 10 can be removed from the vacuumhousing 110 and the guide assembly and ball 320 can be removed from thetransducer assembly 10. The apparatus 100 can then be used to processanother transducer assembly 10.

The apparatus 100 can be configured to process more than one transducerassembly 10 at a time. Instead of the cable 116, it may be advantageousto utilize an actuator rod(s) with an actuator motor to control therotational orientation of the vacuum housing 110. Additionally, whilethe method and apparatus described herein have been directed to atransducer assembly 10 that measures an absolute pressure and utilizes agetter element, etc., the method and apparatus of the present inventioncan also be used to establish a reference pressure in a referencechamber of a wide variety of other gauge-type pressure transducerassemblies.

FIG. 9 illustrates the burn-out, evacuation and sealing process of thepresent disclosure in more detail. In FIG. 9, the x-axis of the processflow represents Time and the y-axis represents Temperature in degreesCelsius. Prior to initiating the burn-out and evacuation process, atStep A of the process flow, the cover 82, ball 320, guide assembly 300and pressure transducer assembly 10 are arranged in the vacuum chamber200 of the vacuum housing 110 and the vacuum housing 110 is rotatedcounterclockwise (forward) as shown in FIGS. 7A and 7B. During Step A→B,over the course of three hours, the temperature in the vacuum chamber200 is raised to a burn-out temperature of 250° C. and the pressure islowered to an evacuation pressure of 10⁻⁸ Torr. After the burn-outtemperature and evacuation pressure are achieved (Step B), the referencechamber 52 is burned-out and evacuated for 20 hours, Step B→C. Shortlybefore Step C is reached, the vacuum housing 100 is rotated clockwise(backwards) to the upright position as shown in FIGS. 8A and 8B. Whenrotated to the upright position, the movement of the ball 320 causes thecover 82 to move towards tube 80 and the two layers of low-temperaturesealing material 94 a to come into contact with each other. Once theburn-out and evacuation step is completed (Step C), the temperature inthe vacuum chamber 200 is raised to 475° C., Step C→D, which causes thetwo layers of low-temperature sealing material 94 a to melt. Step C→Dlasts for three hours. The vacuum chamber 200 is then maintained at 475°C. for 30 minutes, Step D→E, to ensure that the layers oflow-temperature sealing material 94 a sufficiently melt together.Lastly, over the course of 4½ hours, the temperature and pressure in thevacuum chamber 200 are brought to ambient conditions and the assembledpressure transducer assembly 10 is then removed from the vacuum chamber200 of the vacuum housing 110, Step E→G.

By eliminating the intermediate temperature ramp down portion of theprior art method (Step E→F of FIG. 3), which is necessary forming thelow-temperature seal 94 while maintaining the low-temperature seal 98 ina melted state, the burn-out, evacuation and sealing process of thepresent disclosure can be completed in as little as 31 hours. Thus, inaddition to accurately establishing a reference pressure within areference chamber, the present disclosure can also advantageouslyshorten the time that is required to perform the burn-out, evacuationand sealing process of the pressure transducer assembly 10.

Although various embodiments that incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise any other varied embodiments thatincorporate these teachings.

1. A method for attaching a first part to a second part, the methodcomprising: depositing a sealing material on at least a portion of atleast one of the first part and the second part; positioning the secondpart in a space defined by a guide such that gravity tends to pull thesecond part away from the first part; moving the first part, the secondpart, and the guide so that gravity tends to pull the second part towardthe first part; melting the sealing material; and allowing the sealingmaterial to cool.
 2. The method of claim 1, the step of moving the firstpart and the second part comprising rotating the first part and thesecond part about an axis.
 3. The method of claim 1, further comprisingcoupling the guide to a portion of the first part.
 4. The method ofclaim 1, further comprising: providing a mass in the space, the firstpart being positioned between the mass and the second part.
 5. Themethod of claim 4, the mass being a ball.
 6. The method of claim 4, themass comprising either Tungsten Carbide or Silicon Nitride.
 7. Anapparatus for use in joining a first part to a second part, a meltablejoining material being disposed on at least a portion of at least one ofthe first part and the second part, the apparatus including: a chamber,the chamber being moveable between a first position and a secondposition, the chamber being sufficiently large to house the first partand the second part; a guide disposed within the chamber for guiding thesecond part, gravity tending to cause the first part and the second partto separate when the chamber is in the first position, gravity tendingto cause the first part and the second part to be brought together whenthe chamber is in the second position; and a heater for selectivelyheating the chamber to a temperature sufficiently high to melt thejoining material.
 8. The apparatus of claim 7, further comprising: atleast one rotation pin extending from the chamber; a support assembly,the support assembly comprising a support bracket having at least onehole that can accommodate the at least one rotation pin of the chamber;and the chamber being rotatable from the first position to the secondposition when the at least one rotation pin is engaged with the at leastone hole.
 9. The apparatus of claim 7, further comprising: a pressureport defined by the chamber; and a pressure line connected to a pressuresource coupleable to the pressure port, the pressure source beingcapable of establishing a pressure condition in the chamber via thepressure line and the pressure port.
 10. The apparatus of claim 7,further comprising: a weight, the second part being positioned betweenthe weight and the first part.
 11. The apparatus of claim 10, the guidedefining a cylindrical volume and the weight being a ball, the ballbeing disposed in the cylindrical volume.
 12. The apparatus of claim 10,the ball comprising either Tungsten Carbide or Silicon Nitride.
 13. Theapparatus of claim 8, the support assembly further comprising an uppersupport and a lower support, a portion of the chamber resting on thelower support when the pressure housing is in the first position and aportion of the chamber resting on the upper support when the chamber isin the second position.
 14. The apparatus of claim 7, the chamberfurther comprising a cable for rotating the chamber from the firstposition to the second position.
 15. The apparatus of claim 7, furthercomprising an actuator motor and an actuator rod that can be coupled tothe chamber to control the rotational position of the chamber.
 16. Theapparatus of claim 7, the guide having a proximal end, a distal end, andholes located between the proximal end and the distal end.